The present invention relates to position transducers such a graphic tablets and the like for inputting absolute coordinate data to complex control systems and other electronic equipment.
Position input devices such as joysticks, track-balls, and "mice" are commonly used for feeding incremental data to a computer in response to an operator's hand movements, the operator usually adjusting the movements while observing a screen display of the computer. In some important applications, it is desired to feed coordinates that are directly measured from an existing exhibit such as a drawing. In these applications the incremental input devices have major shortcomings regarding both scale factor and position reference.
Accordingly, a variety of graphic tablets have been developed for generating computer position data from a workpiece such as a drawing. Typical graphic tablets of the prior art provide absolute position data relative to a fixed or adjustable reference, the data being generated in response to a stylus that is moved by the operator over the drawing surface. Among the important performance parameters of such transducers are accuracy, repeatability, resolution in both distance and time, range, limiting speed, and ease of operation. Also important are cost, reliability, and compatibility with related systems such as graphic displays.
An important class of graphic tablet incorporates an orthogonal pair of conductor patterns within the tablet. The stylus carries a coil from which a stylus signal is generated. See for example, U.S. Pat. No. 3,975,592 to Carvey, which discloses an array of the conductors in each pattern, the conductors being sequentially energized for producing corresponding electric fields, one of the fields being detected by the stylus for determining a coarse position of the stylus, after which a subset of the conductors is energized at a different rate. In successive cycles of the sequential energization, a sampled counterpart of the stylus signal is integrated positively, then negatively at double amplification. A counter is latched when the integration output reaches zero, the latched counter value being intended to represent the centroid of the detected signal relative to the coarse position. However, latched counter values fail to indicate the real-time centroid of the stylus signal in that computations based on successive samples are employed, the results being subject to error when there is movement of the stylus between samples. Also, cumbersome and expensive sample-and-hold circuitry is required.
Another problem with such systems Is that the detected field strength between the coarse positions is non-linear. U.S. Pat. No. 4,088,842 to Ikedo discloses nonlinear interpolation for detecting intermediate positions of the stylus in a system having the stylus excited and an array of planar pick-up coils in the tablet. Nonlinear interpolators, however, are undesirably complex, expensive to produce, and limited in accuracy. In fact, most electromagnetic coordinate tablets of the prior art require sophisticated signal processing by complex and expensive analog to digital and digital to analog converters, microprocessors, and computer memory. Further, most such tablets use strictly 2-dimensional conductor patterns that are expensive to produce, even with printed circuit techniques. Also, in order to avoid expensive interpolation errors, very fine coarse resolution is resorted to, with consequent added circuit complexity and cost.
It is also known to provide a transmitter coil in the stylus as disclosed in the '842 patent to Ikedo, and in U.S. Pat. No. 4,210,775 to Rodgers et al., which discloses a grid of conductors that is sequentially scanned using a counter for producing an output signal. The leading edge of a pulse that is a filtered counterpart of the output signal halts the counter, thereby producing the position signal.
Another class of coordinate reading devices utilizes magnetorestrictive material as a vibration transmission media. See, for example, U.S. Pat. Nos. 4,216,352 to Chamuel and 4,273,954 to Takeuchi et al. These and similar systems are subject to the limitations of accurately measuring or converting from full scale analog values.
If is often desirable to combine a graphic display capability in a coordinate position transducer. Unfortunately, however, some display devices can be adversely affected by strong electromagnetic fields such as are produced by typical coordinate transducers. Also, magnetorestrictive materials such as used in some coordinate tablets are opaque, thus preventing the use of backlighting, etc.
Thus there is a need for a position transducer that provides high accuracy, repeatability, range and resolution, that is easy to operate, reliable and inexpensive to produce, yet is compatible with low-cost graphic computer and display devices.